CLIMATE CHANGE INCIDENCE ON THE RHONE RIVER HYDROLOGY - TECHNICAL SYNTHESIS LLANES Alexandre
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TECHNICAL SYNTHESIS
CLIMATE CHANGE INCIDENCE ON THE RHONE RIVER
HYDROLOGY
LLANES Alexandre
E-mail: alexandrellanes@yahoo.fr
February 2008
AgroParisTech - Engref à Montpellier
B.P.7355_34086 MONTPELLIER Cedex 4
Tél. (33) 4 67 04 71 00
Fax. (33) 4 67 04 71 01 1
RESUME
Le changement climatique est aujourd’hui considéré comme une réalité par les scientifiques.
Toutefois, les liens de causalité entre changement climatique et modifications des régimes
hydrologiques sont difficiles à établir. A cette difficulté perceptible s’ajoute la complexité
du fonctionnement du régime hydrologique et climatique du Rhône. Dans ce contexte
d’incertitude la société BRL, gestionnaire et distributrice de l’eau du Rhône en
Languedoc Roussillon, s’interroge sur la disponibilité et la vulnérabilité de la
ressource en eau pour les années à venir.
MOTS CLES : Changement climatique, hydrologie, Rhône, étiages, BRL, vulnérabilité
de la ressource.
ABSTRACT
Climatic change is widely considered as a reality by scientists. Nevertheless, impacts of
hydrological extremes are more difficult to observe and to predict. It is shown how the
possible change in the characteristics of precipitations, associated TO climatic change, could
alter the river flows in the basin of the Rhone River particularly for summer and the lowest
water levels. In this uncertain context, the BRL Company, distributor of Rhone water in
Languedoc-Roussillon, investigates the future availability and vulnerability of the resource.
KEY WORDS : Climate change, hydrology, Rhone, lowest water level, availability, growth
samples, BRL, vulnerability of the resource.
ABREVIATION
IPCC : Intergovernmental Panel on Climate Change
MCG : Modèles de Circulation Générale
BRL : Bas-Rhône Languedoc (société)
MEDD : Ministère de l’Ecologie et du Développement Durable
GICC : Gestion des Impacts du Changement Climatique
CNRM : Centre National de Recherches Météorologiques
CNR : Compagnie Nationale du Rhône
SEPALRC : Société d’Etudes et de Promotion pour l’Aqueduc Languedoc-Roussillon-
Catalogne
ENSO : El Niño et Southern Oscillation
MINAN : des débits journaliers minimums annuel
VCN : valeur minimale annuelle du débit moyen sur 7 jours ou sur 30
QCN : valeur minimale annuelle du débit seuil sur 7jours ou sur 30 jours
ETP : Evapotranspiration Potentielle
QMNA5 : débit mensuel minimal dans l’année de période de retour de 5 ans
DOE : Débit Objectif d’Etiage
DCR : Débits de CRise
2
INTRODUCTION ........................................................................................................ 4
1 GENERAL FRAMEWORKS ............................................................................... 5
1.1 GLOBAL CLIMATE CHANGE................................................................................................... 5
1.2 GENERAL CIRCULATION MODELS........................................................................................ 5
1.3 CLIMATE CHANGE AND LOW WATER LEVELS ................................................................... 6
2 THE RHONE RIVER ........................................................................................... 7
2.1 A mode modified during the 20th century ............................................................................. 8
2.2 Low water levels of the Rhone river ....................................................................................... 8
3 IMPACT STUDIES OF THE CLIMATE CHANGE ON THE RHONE RIVER ...... 9
3.1 Retrospective approaches..................................................................................................... 10
3.2. Prospective approaches.................................................................................................... 13
4 CONCLUSION .................................................................................................. 16
3
Introduction
The evolution of the climate related to the human activity is today an established fact (IPCC,
2001). The obvious increase in the concentration of gases and greenhouse effect in the
atmosphere has had effects on a certain number of climatic variables, involving for example
the increase in the number of hot days and a reduction in the number of very cold days.
Precipitation will increase to, at least in the Northern hemisphere, while the snow-covered
peaks and the extent of the non-polar glaciers will be in marked regression. Beyond these
direct observations, the general circulation models (GCM) show a persistence of these
tendencies during the century. Uncertainties which weigh on these projections are very
important because the models are conditioned by socio-economic scenarios imagined for the
years to come. Moreover, the results can significantly vary from one GCM to another and a
certain number of phenomena, acting on intermediate scales, is taken into account in a way
very simplified in the computer codes (of which the space resolution is rather coarse) and
remains sometimes largely ignored. Thus, in horizon 2100, the increase in the total
temperature should range between 1.4 and 5.8 °C.
These climatic disturbances should have effects on the hydrological system. However, these
consequences are difficult to evaluate, for two main reasons: first of all, a total warming can
have antagonistic effects on the flows, by increasing at the same time the evapotranspiration
and precipitations. The influence of these two factors also differs according to the regime,
nival or rain, of the river. Moreover, the space and temporal scales usable at exit of the MCG
do not agree with those of the hydrological model rain/flow. To date, no coherent signal on
large scales was detected for the flows, in spite of changes proven at the regional scales
(Renard, 2006).
Parallel to the context of the total change, the vulnerability link to the extreme hydrological
events is in constant increasing, in particular in France. The water resource is requested
more and more for the needs for agriculture and industry. These last years (severe low water
levels of 2003 and 2005) led to the perception of a recrudescence of the extreme events in
France, which is to date has not been established. The water availability is for the managers
(producer of electricity, water supply, etc) a dominating character as for the evolution of the
plans of stock management. In this context of uncertainty, the BRL company, saddle jib
crane of the water of the Rhone river in Languedoc-Roussillon, wonders about the availability
and the vulnerability of the water resource for the years to come.
This component appears all the more important as a large scale project aiming at extending
their distribution network UPTO Béziers and Narbonne was launched in 2005 (figure 1). That
implies an additional abstraction at peak Source : Sébastien Chazot, BRL
times from 2 to 4 m³/s (BRL, Collectif,
2006). Thus, for the Rhone river subjected
to the modifications of weather forcEs,
which this share of abstractions represents
in hydrologicaimpact l?
The absence of a clear answer on behalf of
the scientists as for the reality of a climate
change affecting the extreme events is
partLy due to the methodological difficulty
to detect tendencies in this type of hydro-
climatic series which, by nature, are
subjected to an important natural
variability. It is however paramount to be Figure 1
able to answer this interrogation, because
as regards prevention of the risks of
shortage, the stationnarity of the
phenomena in time is an assumption which
4is almost systematic.
1 GENERAL FRAMEWORKS
1.1 GLOBAL CLIMATE CHANGE
The general operation of the climate of the Earth is based on the concept of balance of the
energy assessment: the received total radiation of planet must equalize the emitted total
radiation. The distribution of outgoing energy is obviously uniform, neither temporally nor
spatially. This imbalance is the cause of the general atmospheric and oceanic circulation.
The force of Coriolis, related to the rotation of the Earth on itself, complicates this general
outline. There exist thus preferential circulations of energies on the surface of the sphere
(dominant winds, oceanic currents).
In any general information, the climate is obviously not stationary: it has evolved and has
moved since the birth of the Earth and will continue to evolve and move, but one of the major
difficulties in the climate study is the existence of variability which have temporal scales
(annual, decennial, etc), and regional heterogeneous. This low frequency variability is
particularly problematic in the optics of search for evolution. As example, work has put
forward oscillations on the strong rains in Spain and the south of France, by using historical
data (Llasat et al., 2002), but these oscillatory behaviors are only observed and their
constitutive mechanisms remain very largely ignored.
1.2 GENERAL CIRCULATION MODELS
The man influences for this complex system was the increase in the gases that caused the
greenhouse effect which, in short, limits the heat loss of the terrestrial sphere by trapping the
infra-red remission from the ground surface. The increase in these gases does not result
formally in warming. Reality is much more complex. The various components of the climate
cannot be dissociated from to each other and the modification of one of them will be reflected
on the others by feedback phenomenon. It is thus necessary to consider the climatic system
as a whole. It is the role of the general circulation models (GCM), which have as a function to
recreate a realistic climatic evolution under the influence of various forces (natural and/or
anthropic).
A GCM can be regarded as a digital representation of the Earth and processes which control
its climate. For this purpose the planet is discretized in the form of a space division. These
broad total cubes (100 km) or more finely defined regionals (30 km) are characterized by a
whole of variables which describe the various components of the climate: pressure,
temperature, precipitation, moisture, etc. Starting from initial condition data, the evolution of
the climatic conditions under the effect of various forces can be simulated. One of the major
difficulties of the MCG is related to this space discretization: indeed of many phenomena
have a space scale very largely lower than the cutting suggested by the GCM. To try to
describe the operation of the climate and even more its possible evolution under the effect of
various forces is an extremely complex task, being given the great number of phenomena
and scales implied. All the factors are obviously not controlled, so that the use of GCM is
accompanied by several sources by uncertainty:
- The measurement of the climatic phenomena themselves;
- The structure even of the MCG: difference of resolution, parameter setting, algorithms
used, etc;
- Data of forcings, more particularly those related to the socio-economic scenarios for the
future;
- Lastly, the estimate of the impact of the climate change on a variable which is not directly
predicted by the GCM: the flows.
51.3 CLIMATE CHANGE AND LOW WATER LEVELS
Many impact studies of a modification of the climate on the hydrological mode were carried
out during these last decades. Arnell (1999) described the hydrology of the future on the
scale of Europe on meshs of 0,5° by 0,5° by couplin g various possible future climates and a
simplified hydrological model. The results obtained agree on a notable reduction of the
surface water resource in the term 2050 on the continental zones on average latitude and to
the moderate climate: the low water levels are more critical because the increase in winter
precipitations is opposed by an increase in temperatures (and thus of the evapotranspiration)
and a reduction in the pluviometric office pluralities in summer period.
The GEWEX-Rhone experiment and its prolongation within the framework of the program
“Management of the Impacts of the Climate change” of MEDD (GICC) highlight a contrast
seizing between the north of the basin of the Rhone which sees its annual flows amplified
while in the south, the annual throughputs are reduced until - 40% on the basin of Ardeche
(Leblois and Al, 2004). These conclusions must however be relativized taking into account
the low capacity of the models to reconstitute the low water levels of the past.
On the Adour Garonne basin, the Agency of Water in collaboration with the CNRM engaged
a prospective reflexion on the low water levels under climate change (Caballero and Al,
2004). A reduction of 11% of the minimum flows is announced at horizon 2050, consequence
of a reduction in precipitations and an increase marked in the temperatures of the air in
summer.
These few examples of impact studies of the
climate change on the low water levels
presented, although characteristic, do not give
an account of the diversity of the publications,
methodologies, the results and the
controversies specific to this subject. It is
impossible besides to review so much when
the whole of this work is fertile. Figure 2
illustrates this matter well. This multiplicity of
data, in turn complementary or contradictory,
makes the synthetisation of the results difficult.
However, of the whole of these impact studies,
some generic elements can be released
(Sauquet et al., 2007) : Figure 2 : Evolution of the number of
- All the basin slopes do not have the international publications on the set of
same sensitivity to the disturbances of themes “Climate change”. Source: Eric
climatic forces. Indeed, “the initial state”
Sauquet.
of the basin (current mode, low water
level of summer or winter), the geology and the occupation of the ground are
elements which come to moderate the results observed with large scales.
- Uncertainties around the predictions are such as a scenario multiplicities must be
planned to sit the detected tendencies and to give an account of uncertainties
(Prudhomme and Al, 2003). Indeed, the scenarios of Intergovernmental Panel one
Climate Change (IPCC), integrated by an expansion of modelings into large scales
agree all on a rise of the average temperature on the sphere, but the range of the
possible values especially remains broad at horizon 2100.
- Distributed modeling, intellectually satisfactory within a framework of comprehension
of the basin operation, inevitably does not provide the best results in reconstitution of
the low water levels. Consequently, the future trends on this aspect could not be
predicted with accuracy. The alternative is given by a simpler conceptual modeling
which aims mainly a reconstitution of the mode of the catchment area without seeking
to clarify in detail the internal processes. This modeling always remains of topicality in
many impact studies to the detriment of heavier physical models. A contrario, the
6conceptual models will not be able to follow the possible self-adaptations of the
catchment area, not easily appreciable in addition.
2 The Rhone river
The Rhone, by the extent of its catchment area (98 000 km² including 88 0002 km on the
French territory) is subjected to a hydrological mode complexes in connection with the
various climates and crossed reliefs (Figure 3) :
1. The Swiss Rhone
is an Alpine torrent
Figure 3
supplied with water
from the close
mountains. The
mode is marked by
low waters of
winter and rising of
springs and
summer due to the
snow melt and
glaciers.
2. Of its exit of the
lake Léman to Lyon
the Rhone keeps
similar hydrological
characteristics
thanks to the
contribution of the
affluents coming
from Préalpes and
the Jura. Thus the
nival regime
continues to play a
crucial role in the
food of the river.
The rising of springs are marked, but do not make disappear the influence from the
regime of the glaciers on the course of the higher Rhone.
3. In Lyon, the arrival of the Saone river modifies the mode of the Rhone river.
Traversing areas of plains subjected to the oceanic rains, the Saone has an opposite
river regime to the Rhone at the time of its Lyon crossing: high waters of autumn and
winter, low water levels of summer.
4. Downstream from Lyon, thanks to the Saone, the Rhone becomes a powerful and
more regular river. However its hydrological mode is modified at the time of the
crossing of the Rhone-native furrow with the contribution tributary in nival mode
coming again from the Alps.
5. The Cevennes and Mediterranean affluents have an impact only at the time of brief
intense episodes of rising generally in autumn. For the Bas-Rhone river, the severe
risks of low water levels are centered between August and January with one more
critical period between September and October, but the water masses provided by
the glaciers of the higher course prevent this tendency to lead to a true shortage.
72.1 A mode modified during the 20th century
The low water levels which the Rhone can experience are seldom noted. Almost always, the
weakness of the contributions of basin part is compensated by relative abundance due to
another sector of the basin. The Rhone is indeed subjected to a hydrological mode complex
in connection with the various climates and reliefs of the areas coming from its source.
Its mode nevertheless underwent some modifications during the 20th century from the
construction of the great hydroelectric works and reserves on the mainly alpine Rhone and
its affluents (figure 4). The alpine tributary works almost all on the same principle, with the
same objective of hydroelectric production: they store water resulting from the snow melt in
spring and in summer then release it in winter, at the time of the strongest consumption. For
installations of CNR to the current, storage capacities are quasi non-existent. The works
disturb the mode of the Petit Rhone.
Figure 4
Profil en long du Rhône
The period of construction began after the Second World War and was to be completed in in
the 1970s. The service outputs of the tanks increased. Gradually, the mode of the Rhone
changed to reach an operation stabilized in the 1980s. We thus perceive the differences
between the “natural” Rhone observed between 1877-1916 and the Rhone controlled by the
man of the period 1920-2000. There is no significant change of the volumes forwarded on an
annual scale, but a seasonal variation of the flows.
2.2 Low water levels of the Rhone river
The last century knew several remarkable years by their low hydraulicity: 1921, 1949, 1976,
1989, 2003, and 2005. Most exceptional is however 1921 by the duration of the low flows of
the persistent river throughout the year (Duband, Schoeneich, Stanescu, 2004): the flow
threshold of 1000 m³/s. A exceeded in 1921 less than 60 days at the station of Beaucaire,
whereas for one normal year, this average threshold is crossed during 275 days.
A aqueduct project of between the Rhone and Catalonia was studied at the end of the
nineties and for this reason of the studies were carried out on the mode of the minimum flows
of the Rhone to Beaucaire, 10 km upstream of the hydrant of BRL (BRL, SEPA LRC, 1999).
8in Beaucaire The Rhone drains a basin of 95590 km2, of which 85243 km2 are located in
France (15% of the French territory). From 1971 and 1997, with on average 54 billion m3
annually run out, the Rhone presents thus nearly one the third of the metropolitan territory
sea contributions.
The modes of current low water levels of the Rhone river are well encircled thanks to an
exploitation of quality of the hydrometric station of Beaucaire, which can be regarded as
presenting the best possible precision in hydrometry of river, that is to say a knowledge with
better than 5-10% close to the low water levels.
The hydroelectric influences are inventoried, traced exhaustively since 1949 on the basin;
methodological difficulties do not allow to desinfluence the hydroelectric use of the flows of
the Rhone river in Beaucaire with a step of time lower than the month. Broadly the
hydroelectric influence is of annual a net taking away of 95 m3/s (derivation of the Durance
towards the pond of Berre).
It is a carryforward of the flows from April at August in winter and spring. With the autumn,
the influence is neutral in average year, always positive during the marked low water levels
(support of low water level from + 50 to + 200 m3/s on all the low water levels of 1971 to
1997).
Le Rhône à Beaucaire : débits minimums enregistrés de 1970 à 2005
The low water level of
the Rhone is Figure
1000 5
Source : Sébastien Chazot, BRL
established from 900
August to January, 800
October and 700
September being most 600
critical, more
m3/s
500
particularly October for 400
the situations of 300
prolonged dryness 200
(see figure 5). The 100
VC30 (lower medium 0
flow during 30
1/1
1/2
1/3
1/4
1/5
1/6
1/7
1/8
1/9
1/10
1/11
1/12
consecutive days)
(return period: 50
years) is established respectively with 390 m3/s for the year, 490 m3/s over August-
September; the VC5 (T= 50 years) is 320 m3/s for the two situations.
With the sight of these values, the 3 m3/s taking away point under consideration for
the extension of network BRL until Narbonne (BRL, Collective, 2006), represent
approximately of 1% of the values characteristic of severe low water levels of the
Rhone; this is to be added to the current taking away of 15 m3/s, that is to say on the
whole, 6% of the values of severe low water levels.
3 Impact studies of the climate change on the
Rhone river
The modification of the Rhone hydrological mode induced by a climatic drift is not easily
perceptible: the dubious results (sometimes contradictory), the many variables of studies,
ambiguities of the exploited models, attest difficulty of this research. Thus, various
methodologies of approaches are in the middle of the current thinking since it is of their
improvement that the accuracy depends on the future predictions.
The methods of investigation propose to explore the recent past to detect possible
modifications of the hydrological behavior of the basins ascribable slopes to the increase in
gases with greenhouse effect and the future while being based on climatic scenarios to
characterize the response of the Rhone river to the modifications of climate forcings.
9The first point includes/understands analyses of stationariness carried out on the samples of
descriptive variables of the low water levels, it is a retrospective approach.
The second point falls under the traditional impact studies. This is carried out using total
hydrological rain-flow models with a chock optimized on the criteria of resource of the
catchment area of the Rhone river. The model rain-flow is a tool which makes it possible to
simulate the flows in a point given of a river starting from the knowledge of liquid or solid
precipitations on the catchment area corresponding and of the potential return to the
atmosphere via the phenomena of evaporation. It is a very simplified representation of the
operation of the catchment area and the intrinsic complexity of the hydrological processes.
These models are then forced by various representations of the future climate placed at the
free disposal of the scientific community. These forces are examined to measure the future
sensitivity of the characteristics of low water levels. It is a prospective approach.
3.1 Retrospective approaches
These approaches aim at exploring the recent past to detect unstationarities in the flows of
the drynesses. Indeed, like any climatic size, the hydrological variables know an intrinsic
temporal variability. This variability is usually synthesized by a statistical distribution adjusted
on the last achievements (hydrometric data). A climate change would cause a revision of the
law established or the use of another. Except major disturbance, the new range of possible
values would be for a great part recovering with the old one: the frequent values become a
little more or a little less probable. The hydrologist statistician very often seeks a proof of an
evolution in negligible variations.
Detection of non-stationarity in low flows
A complete sequence of more than 40 years is necessary to integrate a representative
sample of the alternation of the dry and wet years (Sauquet & Haond, 2004). Such series are
unfortunately rare. The longest chronicles have very often related to basin slopes having
undergone human actions for several decades.
With the natural signal is grafted a anthropic disturbance which is advisable to identify and to
filter before concluding on the origin from a possible drift.
Principle of the test
There exist many tests of search for nonstationariness in the hydrological modes, this one
was carried out by the research team of the Lyon CEMAGREF associated with CNR
(Sauquet et al., 2005).
The statistical approach brings a decision-making aid concerning an assumption. The tests
rest all on a variable of decision or statistical T which one knows the distribution under the
assumption HO. One can then define the terminals of an interval of the values which the
statistics can take if H0 is true and to locate the value T at it taken for the analyzed sample. If
T is apart from the interval, the value is considered to be not very plausible, therefore H0 is
false.
The calculations carried out on the samples will result in retaining the assumption of
stationnarity HO or its H1 alternative knowing that only one is true (Sauquet et al., 2005).
See the table below (source conference E. Sauquet Engref 2007) :
10Reality
H0 H1
Accepted H0 whereas H0 is true Accepted H0 whereas H1 is true
Results of the test
Accepted H1 whereas H0 is true Accepted H1 whereas H1 is true
To defer the results of the test on a diagram (figure 6), one must choose the variables such
as :
- Z relating to the descriptive variables of the hydrological mode extracted to the long
observation series
- Evolution in time.
One tests the H0 assumption which represents a stationary series.
Z Z
t
OU
The slope t
Rupture
time time
Figure 6 The difference T between average from and
(source : conférence E. Sauquet Engref 2007) of other of a date in supposed rupture
In these two cases, the goal is to test if T ≠ 0, significantly, i.e. apart from a fixed interval of
tolerance.
International results
In Canada, Zhang et al. (2001) insist on the existence, the sign and the origin of the drift of
the monthly medium flows: the rise of the temperatures of winter end involves earlier snowy
coat, from where flows increased March and April and reduced in May and June. For the
United States, there exists a consensus on the low water levels which from now on would be
marked (Douglas et al, 2000). The detected tendencies could be allotted to the persistence
of the same phase of the Atlantic Northern Oscillation during the last years. In Oceania, the
difficulty comes from the marked influence of the phenomenon ENSO, that prints a strong
interannual variability on the flows and masks the possible drifts. The low water levels less
marked during the three last decades observed in the Island of the South of New Zealand
would be primarily due to a greater frequency of the phase El Niño over this period (Mac
Kerchar and Henderson, 2003).
In Europe, Hisdal et al. (2001) are interested in the low water levels, but do not detect
anything really tangible, because of the anthropic disturbances, of the short observation
periods and natural variability.
A total conclusion is obviously not allowed taking into account the diversity of the examined
variables, the operated pretreatments, the periods considered and the tests applied.
11Rhone river flows
The French part of the Rhone was the subject of several investigations of the Lyon
CEMAGREF and CNR concerning the stationnarity of the flows (Sauquet & Haond, 2004).
The stations selected for this examination are famous being slightly influenced, at least for
high waters, nothing is not sure concerning the low water levels.
The stations of the high Rhone present a similar behavior: a reduction of the daily outputs
minima annual (MINAN) and an increase in the annual minimal value of the medium flow
(VCN, over 7 days or 30 days) and of the annual minimal value of the flow threshold (QCN,
on 7jours or 30 days). It is necessary to notice the coherence of the dates of ruptures located
essentially in the years 1970. Downstream, it is the station established in Perrache (Lyon),
which is characterized by the number of stationnarities.
The stations of the Bas-Rhone (Valence and Beaucaire) seem to post a stationary mode. Of
these analyses, the absence of detection of rupture at the known dates marking the
installation of the great works and derivations is a surprise. These disturbances are added or
neutralized those possibly generated by an evolution of the climate.
In the North of the Alps, in winter, the flows are low by retention of water in the form of snow.
Nonstationarities concentrate in this sector with a significant evolution towards less severe
low water levels in winter. Is it necessary to see there the consequence of an increase in the
temperatures of the air? Nothing is still acquired, it should as a preliminary be made sure that
the flows indeed natural or are influenced very little. For the southern part of the prospected
zone, anomalies could be identified for certain stations and on certain variables, but they are
not sufficiently many and coherent to conclude (Sauquet et al. 2005).
In short, it comes out from this work an absence of real sign of disturbance on the
French part of the Rhone. Tests applied to several variables characterizing various
aspects of the hydrological mode made it possible to confirm international results.
Anomalies could be identified for certain variable stations and unquestionable, but
they are not sufficiently many and coherent with supposed dates of beginning of the
climate change to charge them to the intensification of the greenhouse effect.
Moreover, the evolution that they print registers in a natural variability of the
hydrological mode. Lastly, they can be explained by human actions.
Scandinavian Approaches applied to the Rhone river
Other research, aiming at apprehending the signature of the climate change in the
Scandinavian hydrological modes, was undertaken on the seasonal variation of the flows
subjected to climatic forcing (Krasovskaia, Gottschalk, 2002). According to atmospheric
forcing, a hydrological mode can change and present a rather different seasonal diagram,
but the intensity of the awaited disturbance depends on the sensitivity of the hydrological
mode concerned. Under the conditions prevailing in Scandinavia, the fluctuations of the
average annual temperature, even modest (±1°C), are reflected at the same time in the type
of mode and its hydrological stability in approximately 20% of the studied cases
(Krasovskaia, Gottschalk, 2002).
The modes of flow can be considered under the angle of their intrinsic dimensions and
fractales, i.e. seen like as many characteristics and components subjacent with a given
hydrological mode (size, geological nature and slope of the catchment area, precipitation,
etc). These dimensions give a convenient characterization of the stability of each
hydrological mode: the most stable types for example (presenting little interannual
variability), are those having the lowest dimensions.
These Scandinavian approaches were applied to the Rhone (Krasovskaia, Gottschalk and
Leblois, 2002). This methodological study aims at detecting an echo of the climatic
fluctuations in the flow seasonal variations, through a sample of 140 chronicles of flow to
steps of monthly times of 1975 to 1990 (thus 2240 year-stations of twelve values each one).
For this study, only the oceanic and nival rain modes hydrological, whose seasonal behavior
is opposed by the high ones and low waters, were considered.
12Without extending on methodological complexity from the space and statistical processing
from various dimensions from the Rhone hydrological mode, it should be retained that the
comparison of the results for the various periods observed watch a clear evolution of the
mode towards a greater irregularity in the seasonal variation of the flows.
The modes of transition, in very large proportion already, increased towards the end
of the year 1980, with depend on the nival modes. The increase in the modes of
transition (pluvio-nival mode and mode Mediterranean characterized by autumnal
secondary high waters) means concretely that maximum and the minimum annual
ones of the flows could be observed more frequently than maintaining at other periods
of the year than those awaited. As the flows of transition are of greater dimension
(unstable) that rain or nival modes pure, there could be a difficulty increased with the
forecast of the modes of future flows.
In short, we will witness a gradual reduction in the nival modes to the profit of the rain
modes. These results are coherent with those obtained for Scandinavia (Krasovskaia,
Gottschalk and Leblois, 2002).
3.2. Prospective approaches
GICC Rhône Project
The project of Management of the Climate change Impacts (GICC) gathers several scientific
partners, organizations and industrialists: BRGM, CEMAGREF, CNRS, EDF, Weather-
France. The objective is to give a lighting on the impacts to be envisaged on the French part
of the Rhone river basin, because of climate change resulting from a doubling from CO2 in
the atmosphere (probable expiry 2050). The project was declined in two work phases:
- Climatic scenarios and their hydrological consequences
- impacts relative to various physical, biological and socio-economic fields.
The first phase of project GICC the Rhone river began in 1999 (Leblois et al., 2004). For this
study, six configurations resulting from 4 MCG were considered (LMD of IPSL, ARPEGGIO
of Weather-France, HC of Hadley Center and UR of the University of Reading). These
configurations correspond to simulations with CO2 doubling under assumption of 1% of
annual growth.
This climatic impact study is based on the achievements of the co-operative project the
Gewex-Rhone that led to the installation of an unequalled database on the basin and to the
development of a hydro-meteorological modeling of increasing complexity over one long
period of time (1981-1996) (Etchevers et al., 2001). This base constitutes the reference of
the climate present to evaluate the disturbances of the various compartments of the
hydrosystem under assumption of the climate to modify.
The 6 configurations are divided into 4 configurations “low resolution” (2 or 3 points MCG
only on the basin) and 2 configurations “high-resolution (between 10 and 28 points on the
basin).
Generally, the MCG have a good capacity to simulate the monthly cycle of the temperature
of the air, but have difficulties in represent monthly average precipitations with monthly
variations being able to be important.
From the results of these MCG, the coupling of a model calculating evapotranspiration
(ISBA) and of a model rain flow (MODCOU), makes it possible to clarify a certain number of
impacts of the climate change on the cycle of water.
Hydrological impact on the basin of the Rhone starting from model ISBA-MODCOU
Model ISBA-MODCOU is distinguished from hydrological models simpler by a description
rather fine of the processes of surface, in particular related to the resolution of the diurnal
cycle of the energy assessments and hydrous of surface. This model highlights the effects of
the climate change on the assessments of surface and each effect starts various processes
closely connected :
131. 1. The increase in the winter rains stronger than the increase in the ETP increases
the water contents of the ground, therefore streaming of surface and drainage. In
mountainous area, the simultaneous increase in the temperature of the air is
accompanied by a reduction in snow-covered precipitations and consequently by a
reduction of winter accumulations.
2. The reduction in estival precipitations increases the hydrous deficit of the ground
which is reinforced by the increase in the ETP (increase in the temperature of the air),
which involves a desaturation of the grounds much earlier and more severe low water
levels in autumn.
3. The increase in the temperature of the air involves an increase in real evaporation
during the winter and spring because the grounds are well fed out of water. On the
other hand, real evaporation tends to be reduced in summer and autumn by effect of
hydrous stress. The significant heat flow towards the atmosphere is then reinforced.
4. Snow-covered surfaces decrease on average by 25% to 40% following the
configurations. The snowy coat for high altitudes is a little less affected because the
average temperature is largely lower than the melting point there. The anomalies of
the snowy coat are likely to very strongly impact the mode of the alpine rivers. The
early nival cast iron involves a peak of raw in May, in approximately one month
advance. It is remarkable to note that this shift is present in all the scenarios. It
follows low water levels much more marked from there in July and August.
Results
The regional variations of the hydrological impacts can be strong within the basin of the
Rhone. The impact on the flows is very variable between North (the Saone) and the South of
the basin (Ardeche, Durance): increase in the annual throughput of the Saone of
approximately 10%, stronger reduction on the basins of Ardeche and the Durance (reaching
15% for this last). These contrasted regional answers are compensated on the level of the
discharge system of the Rhone to the Fish ponds which presents a weak reduction of the
annual volume of water (approximately 2%). The real evapotranspiration is strongly
increased on the zones of relief because of the reduction of snow-covered surfaces.
Evaporation increases also slightly in North and is reduced much more strongly (5%) in the
South under the effect of the hydrous stress of the ground. The water contents are
decreased overall on the whole of the field (because of the increase in evaporation),
manifestly on the basins of the South accentuating their current vulnerability thus (Leblois et
al., 2004).
To conclude on the results from project GICC, the impact of the climate change is
adjusted seasonally. The flows on the Rhone river basin are likely to decrease by May
to November. The surfaces covered with snow during the winter decrease on average
by 25 to 40% following the scenario. The cast iron of the snowy coat occurs earlier
and snow-covered precipitations decrease. The strong spring flows are generally
reduced and they appear earlier (1 front month). The winter flows increases
appreciably (more winter rains), whereas the summer flows are reduced by 50%.
These general tendencies are reproduced by all the scenario, to differing degree, but
no precise prediction of quantity could be advanced.
The methodological uncertainties and limits are still too important for the tangible
assertion of quantity of water run out by 2050.
Extrapolation with other basins
This model is currently places from there on the basins Adour-Garonne and the Seine, on
which hydrological simulations of the current period are satisfactory with the decadal steps of
time. These two basins have hydrological modes rather different from those of the Rhone
which can lead to more marked vulnerabilities vis-a-vis the climate change. A study on the
basin Adour-Garonne, watch which the hydrological sensitivity to the climate change can be
in particular increased for the modes of low water levels (Caballero and Al, 2004). Indeed,
14the nival component would be more affected than on the basin of the Rhone because of the
lower altitude of the Pyrenees combined with a more moderate increase in winter
precipitations.
For the small basins, the problem is more difficult still because of the difference in space-time
scales between scenarios MCG and these small basins (a few km2) in time of prompt
response (a few hours). For the basins of the Mediterranean coastline, one can imagine that
the climate change will be accompanied by winter and autumnal risings stronger (the
increase in the intensity of the rains is all the more appreciable as the size of the basin is
low) and of periods of dryness more marked even (Redaud et al., 2002).
Limits of the study
Like any scientific approach, the GICC-Rhone study highlighted some limits of its own
methodology which are as many future ways of research. Method employed lack of
relevance for the extremes, this being less the fact of the limitation of adopted hydrological
modelings, that initial choice of the technique of adaptation of scales of the atmospheric
scenarios which appear privileged the average flows. Indeed :
- They are the fluctuations of the monthly median values which are deferred on the
chronicles of the atmospheric variables to the steps of time tri-schedules or days laborer.
- the MCG are recognized like imperfect, but their sensitivity to the CO2 doubling is regarded
as Juste.
These two effects combine to make debatable the fluctuations of the variables modelled, in
particular of the hydrological variables and their extremes (Leblois et al., 2004).
Risque-Décision-Territoire 2003 Project
The purpose of this approach does not call into question the results of project GICC, but is to
refine the conclusions of the impact studies on the low water levels of the Rhone and to open
a parallel way with different tools and data to appreciate uncertainties while following a
specific objective; a diagnosis on the threats which weigh on the surface water resource
(Sauquet et al., 2005). These studies were undertaken on basins slopes feeding the French
part of the Rhone-native basin.
- It is a standard step of impact study to the regional scales (Casing et al., 1994):
- Choice and chock of a model rain-flow on several affluents of the Rhone river under
the last conditions.
- Extraction of climatic variables of the future years and selection of the relevant data
for the Rhone sector and its affluents. It is the description of the climate resulting from
the MCG.
- Construction of future time serieses starting from the monthly anomalies delivered by
the climatic models.
- Application of the model rain-flow selected supplied with the series representative of
the possible future climates;
- Examination of the modifications generated on the hydrological mode by comparison
with the observations.
Numerical models
For these studies, the basin is regarded as a homogeneous entity. Four versions of models
rain-flow were tested and fixed on phenomena observed: GR4J, HBV, TOPMODEL and
IHACRES. Four instrumented basins of the French part of the Rhone little influenced by
snow were studied: Eyrieux, Drome, Roubion and Azergues.
The models are fed by chronicles of rain of basin, obtained by simple arithmetic mean of the
actual values at the points of measurement. There is no universal model which is appropriate
for all the basins, thus each basin has a preferential model.
These models rain-flow are then forced by the MCG relative to the various climatic scenarios,
i.e. by disturbed series (Sauquet et al., 2005).
15Impacts on the mode of low waters
One of the assumptions is that the transformation rain-flow will be modified little by 2080
(absence of adaptation of the area catchment in terms of occupation of the ground,
vegetation and evolution of the taking away of water). The future characteristic flows were
extracted: Q90 (flow exceeded 90% of time) and QMNA5 (minimal monthly flow in the year of
period of 5 years return).
Indicators of brittleness of the low water levels and critical point of the system were worked
out for this research (Sauquet et al., 2007). These indicators all are with the fall whatever the
scenario, this fall is sometimes drastic. A more operational manner to exploit these
simulations is to evaluate the perenniality of the system by applying current rules of
management to the future context (Sauquet et al., 2007). The crisis situations (failures of the
system) correspond to sequences of flows lower than specified values for which a
priorisation of the taking away is necessary. Within the framework of a plan of management
of the low water levels, these thresholds could be defined by the DOE (Objective Flows of
Low water level) or the DCR (Flows of Crisis). To establish a diagnosis, one can be based on
the concepts of reliability and impact strength which respectively qualify the probability of
observing the system in a satisfying state and the capacity of the system to return in a
satisfactory state (Loucks, 1997). These indicators must be used for comparing various
policies of management of water and appreciating the durable character of the system in
time (Peters et al, 2005).
Within the framework of this study, these indicators were included for the basin of the Rhone.
All in all, Sauquet et al. (2007) note very little difference between the studied basins. Thus,
management will be problematic and this in spite of the conservation of the same flow of
objective low water level. The crisis situations could be observed more than 10% of time. The
durations of the episodes would be lengthened (an anticipation of the crises would be thus
more necessary). Finally, the performances would be reduced with stronger deficits.
Here still, only of the tendencies can be released. Whatever the climatic scenario, a
tendency to an increased severity of the low water levels seems to take shape. The
potentialities of refill supported by more abundant, beneficial winter precipitations for
the river in low water level, are neutralized by a rise of the estival temperatures.
Finally the assessment is negative for the river in term of resource. However, the
ranges of reduction are very diverse according to the basin considered.
4 Conclusion
What is necessary to conclude from it? Such as they are, the various climatic scenarios
cannot answer the impact studies alone. The hydrologists must ensure a day before on these
scenarios, but the current results are still, and for a long time, too dispersed to establish
solids conclusions in quantitative terms for the future. Moreover, there does not exist any
coherence with large scales. For the catchment area of the Rhone, no conclusion
relating to the quantities of water available in the future, nor even concerning the
general response of the basin to the climate change, can be advanced.
Various approaches were developed to apprehend the incidence of the climate change on
the hydrology of the Rhone:
- First of all, retrospectively, by statistical analysis, the search for variability in the low
water levels observed since approximately a century. It arises, in spite of the
anomalies of certain stations, an absence of real sign of disturbance. With a
methodological approach different from the treatment of long series of flows, other
research concludes on a tendency to a greater irregularity in the seasonal variation
from the flows.
- Then, approach GICC the Rhone insists on the regional variations of the hydrological
impacts within the basin of the Rhone. Indeed, the impact on the flows is very
variable between North and the South of the basin and the increase in the
16evapotranspiration overall will decrease the quantities of water to the discharge
system.
- Lastly, the researchers working on project RDT 2003, use a certain number of flow
indicators characteristic and conclude with the drastic fall of those.
The clarification of these results and the prediction of the quantities available in the
future depend closely on methodologies used, in particular of the improvement and
the refinement of the MCG, but also of the choice of a method adapted to a local
context. It then appears essential to specify that, in the actual position of knowledge,
no figure can be advanced as for the low water levels future and only of the
tendencies can be released. These tendencies themselves always do not go in the
same direction, which pushes us to conclude on the difficulty, even impossibility of
answering the problems evoked in introduction.
The climatic risk is, with final, not easily appreciable, it must thus be supplemented by a
concept of vulnerability to establish independently or almost evolutions of the climatic
scenarios, on each basin or under-basin. This concept of vulnerability of the low water levels,
or indicator of brittleness of the low water levels, the competence of the hydrologist must be
a signature of the revealing basin of the process operating the basin in terms of
transformation rain-flow.
Moreover, the behavior of the basin around its point of current operation is what interests the
hydrologist and the manager for an effective management in the medium term. It is a
question of being centred on studies of sensitivity and of concentrating on joint realistic
evolutions of the key variables conditioning the low water levels.
A new project carried out by the CEMAGREF and CNR will make it possible to establish a
typology of basins for which the contributions are most likely to be decreased and for which
the associated risks of shortage of water are increasing. They also plan to work on a scale
them under-basins to try to isolate the local sensitivity of the sector from that of the sectors
upstream. A reactualization of the scenarios and a better strategy of space-time
disintegration are to be considered.
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